It is known to produce polyisocyanates having a biuret structure by directly reacting excess quantities of organic diisocyanates with organic diamines at elevated temperatures. Thus, DE-OS 3,114,638 describes the reaction of excess quantities of a mixture of 80% 2,4- and 20% 2,6-diisocyanatotoluene with alkylphenylenediamines, wherein the reactants are reacted within a period of 3 hours at 100.degree. C. with a zinc acetonyl acetate catalyst. While this approach, i.e., producing biurets with the addition of a catalyst, does indeed require only low temperatures, it would be preferable in practice, where possible, to avoid using a catalyst since, in order to terminate the reaction, the catalyst must then be inactivated with benzoyl chloride.
EP 0,003,505 describes an industrially practicable method for aliphatic isocyanates. In this process, the diamine is introduced into the initially introduced diisocyanate by means of a smooth jet nozzle of defined dimensions using elevated pressures. One disadvantage is that, when the described smooth jet nozzle is used with aromatic isocyanates under the conditions described in the Examples, it is not possible to suppress the formation of solids in the form of precipitated urea. For this reason, in the case of isocyanates, a urea dispersion is first purposefully produced in a preliminary stage at low temperatures of up to 120.degree. C. so that it may subsequently be converted into the biurets by heating.